According to researchers at the Harvard-Smithsonian Center for Astrophysics (CfA), we might soon be able to detect hints of technologically advanced alien civilizations by measuring high levels of polluting gases in the atmospheres of distant exoplanets. The approach should become viable soon after the James Webb Space Telescope (JWST) is launched in late 2018.

The new and more powerful space telescopes set to replace Hubble in the coming years will be able to uncover new details on very small and very distant celestial objects throughout the universe. In particular, the JWST will make observations mainly in the infrared spectrum, which is ideal for targeting exoplanets and, through spectral observations, even pinpoint which planets might be (to the best of our knowledge) the best candidates for hosting extraterrestrial life.

Using the limited data at our disposal, scientists have recently speculated that as many as one hundred million exoplanets in our galaxy alone may host complex forms of life. But if life does indeed exist outside of Earth, how could we tell from a distance whether it is a primordial bacterial form or a highly advanced technological civilization?

Researchers at the Harvard-Smithsonian Center for Astrophysics have suggested a possible method to do just that. In a recently published study, they have concluded that the JWST will be able to spot the fingerprints of specific atmospheric pollutants which, if found in meaningful concentrations, would reliably indicate the presence of a technologically advanced civilization.

Though seemingly far-fetched, the authors argue that this approach has merit. We expect the atmosphere of life-bearing planets to contain oxygen and methane in abundance, but their detection would not be a conclusive sign of intelligent life. By contrast, some chemicals causing atmospheric pollution are purely artificial and so, according to the scientists, they should be interpreted as a reliable sign that technologically advanced little (not-so-) green men are close-by.

Harvard student Henry Lin and professor Avi Loeb estimate that the JWST should be able to detect tetrafluoromethane (CF4 and trichlorofluoromethane (Ccl3F), two types of chlorofluorocarbons (CFCs), if they are present in concentrations approximately ten times higher than on Earth. These two chemicals are found in aerosols and chemical solvents and have been known to cause the thinning of the ozone layer in our own atmosphere.

"It is ironic that high concentrations of molecules with high global warming potential (GWP), the worst-case scenario for Earth's climate, is the optimal scenario for detecting an alien civilization, as GWP increases with stronger infrared absorption and longer atmospheric lifetime," say the authors.

But the researchers note that the molecules could also be there not as harming and unwanted pollution, but also, and perhaps more likely, as a way to terraform a planet on the edge of the habitable zone.

"Targeting pollutants like CFCs is ideal, as they are only produced in significant quantities by anthropogenic activities," say the authors. Moreover, the gases could remain in the atmosphere for tens of thousands of years, which increases our chances to spot such a planet even if the civilization had annihilated itself.

Unfortunately, the JWST will only be powerful enough to detect such pollutants on Earth-like planets orbiting white dwarf stars, which are incredibly dense stellar remnants of Sun-like mass and Earth-like size. However, these systems would be an interesting place to look for life, since we have already found a number of promising exoplanets in similar environments.

With JWST, a few hours of integration time will be enough to detect Earth-like levels of water vapor, molecular oxygen, carbon dioxide and other generic biosignatures on planets orbiting a white dwarf; beyond that, observing the same planet for up to 1.7 days will be enough to detect the two CFCs in concentrations of 750 parts per trillion, or 10 times greater than on Earth.

Though admittedly a long shot, this method is more easily achievable than other approaches that have been advanced in recent years, such as looking for city lights, which would demand optics well outside our current capabilities (you'd need a mirror 100 times larger than Hubble's just to detect lights on the planetary system closest to our own). It also doesn't rely on the alien civilization having already found us, as is the case with the more "conventional" method of scouring the electromagnetic spectrum for alien transmissions.

The new method should already become viable for white dwarf systems in five years' time as the JWST goes online. After that, we will most likely be able to expand our search to Earth-like planets orbiting Sun-like stars with the very next generation of space telescopes.